G-SYNC 101: G-SYNC vs. Fast Sync

The Limits of Single Frame Delivery

Okay, so what about Fast Sync? Unlike G-SYNC, it works with any display, and while it’s still a fixed refresh rate syncing solution, its third buffer allows the framerate to exceed the refresh rate, and it utilizes the excess frames to deliver them to the display as fast as possible. This avoids double buffer behavior both above and below the refresh rate, and eliminates the majority of V-SYNC input latency.

Sounds ideal, but how does it compare to G-SYNC?

Evident by the results, Fast Sync only begins to reduce input lag over FPS-limited double buffer V-SYNC when the framerate far exceeds the display’s refresh rate. Like G-SYNC and V-SYNC, it is limited to completing a single frame scan per scanout to prevent tearing, and as the 60Hz scenarios show, 300 FPS Fast Sync at 60Hz (5x ratio) is as low latency as G-SYNC is with a 58 FPS limit at 60Hz.

However, the less excess frames are available for the third buffer to sample from, the more the latency levels of Fast Sync begin to resemble double buffer V-SYNC with an FPS Limit. And if the third buffer is completely starved, as evident in the Fast Sync + FPS limit scenarios, it effectively reverts to FPS-limited V-SYNC latency, with an additional 1/2 to 1 frame of delay.

Unlike double buffer V-SYNC, however, Fast Sync won’t lock the framerate to half the maximum refresh rate if it falls below it, but like double buffer V-SYNC, Fast Sync will periodically repeat frames if the FPS is limited below the refresh rate, causing stutter. As such, an FPS limit below the refresh rate should be avoided when possible, and Fast Sync is best used when the framerate can exceed the refresh rate by at least 2x, 3x, or ideally, 5x times.

So, what about pairing Fast Sync with G-SYNC? Even Nvidia suggests it can be done, but doesn’t go so far as to recommend it. But while it can be paired, it shouldn’t be…

Say the system can maintain an average framerate just above the maximum refresh rate, and instead of an FPS limit being applied to avoid V-SYNC-level input lag, Fast Sync is enabled on top of G-SYNC. In this scenario, G-SYNC is disabled 99% of the time, and Fast Sync, with very few excess frames to work with, not only has more input lag than G-SYNC would at a lower framerate, but it can also introduce uneven frame pacing (due to dropped frames), causing recurring microstutter. Further, even if the framerate could be sustained 5x above the refresh rate, Fast Sync would (at best) only match G-SYNC latency levels, and the uneven frame pacing (while reduced) would still occur.

That’s not to say there aren’t any benefits to Fast Sync over V-SYNC on a standard display (60Hz at 300 FPS, for instance), but pairing Fast Sync with uncapped G-SYNC is effectively a waste of a G-SYNC monitor, and an appropriate FPS limit should always be opted for instead.

Which poses the next question: if uncapped G-SYNC shouldn’t be used with Fast Sync, is there any benefit to using G-SYNC + Fast Sync + FPS limit over G-SYNC + V-SYNC (NVCP) + FPS limit?

The answer is no. In fact, unlike G-SYNC + V-SYNC, Fast Sync remains active near the maximum refresh rate, even inside the G-SYNC range, reserving more frames for itself the higher the native refresh rate is. At 60Hz, it limits the framerate to 59, at 100Hz: 97 FPS, 120Hz: 116 FPS, 144Hz: 138 FPS, 200Hz: 189 FPS, and 240Hz: 224 FPS. This effectively means with G-SYNC + Fast Sync, Fast Sync remains active until it is limited at or below the aforementioned framerates, otherwise, it introduces up to a frame of delay, and causes recurring microstutter. And while G-SYNC + Fast Sync does appear to behave identically to G-SYNC + V-SYNC inside the Minimum Refresh Range (<36 FPS), it’s safe to say that, under regular usage, G-SYNC should not be paired with Fast Sync.

Should “Reduce Buffering” option in Overwatch be enabled or disabled? Many competitive/pro players suggest having reduce buffering on to get higher framerate and reduced input lag but would having this option on have a negative effect on G-SYNC?

Also I recently upgraded my system to a i7-8700k and 1080ti. I usually sit at a steady 300fps on Overwatch now and use a 240hz monitor (Asus PG258Q). Would G-SYNC be worth using in this case?

“Reduced Buffering” is effectively Overwatch’s version of Nvidia’s “Maximum pre-rendered frames” set to “1,” so yes, leave it enabled for the lowest input lag, and no, it does not interfere with G-SYNC functionality; it was enabled for all the tests in this article. As for 240Hz G-SYNC w/238 or 237 FPS limit vs. 240Hz V-SYNC OFF w/238 or 237 FPS limit, the lag difference is almost zero; see the 240Hz chart on this page here. And for 240Hz G-SYNC w/238 or 237 FPS limit vs. 240Hz V-SYNC OFF at 300 FPS, you’re only looking at a 1ms difference in lag,… Read more »

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5 months 18 days ago

Member

pervyjutsu

So I’ve been experimenting with the different kinds of settings: – 240Hz G-SYNC w/ 238 or 237 FPS limit – 240Hz V-SYNC OFF w/ 238 or 237 FPS LIMIT – 240Hz V-SYNC OFF w/ 300 FPS limit I actually found myself liking 240Hz V-SYNC OFF w/ 238 or 237 FPS LIMIT the best. Up until now I have always used 300 FPS limit because that is what everyone said was best input lag-wise, but after trying the lower 238 or 237 limit today, wow I can aim so much better for some reason? It is definitely not placebo because I… Read more »

We always have to consider the (never to be underestimated) placebo effect, but… Like I mentioned in my last reply, the unique way each of those various scenarios deliver frames could be causing subtle differences in lag patterns that may affect aiming; muscle memory, conditioning, subconscious processes, etc. Because quite frankly, unless Overwatch is doing something abnormal with a 300 FPS limit above your refresh rate (which none of my existing tests show), or you’re experiencing a system specific issue, then the scenario you’re claiming feels more responsive, is actually slightly less responsive, at least where raw average input lag… Read more »

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5 months 17 days ago

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bcbuse

First, this is the best Gsync/Vsync information on the internet. I appreciate the effort you put into this, well done.

I read a comment you posted somewhere that ‘technically’ the absolute least input lag would be with Gsync Off + Vsync Off + Framerate upcapped(getting at least 2x the monitor refresh rate). Can you approximate how much less input lag that would be versus Gsync On + Vsync On(NVCP) + Framerate capped 2 below monitor refresh rate?

Depends on the maximum refresh rate and how high the framerate is sustained above it, but at 2x ratio, and at higher refresh rates, you’re looking at only 1-3ms less average input lag with G-SYNC off + V-SYNC off + FPS uncapped vs. G-SYNC on + V-SYNC on + -2 FPS limit when measured at crosshair level.

You can see the worst case first reaction differences between those two scenarios on the ninth page here.

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5 months 19 days ago

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daniel8747

to clarify: with fps capped 2-3 fps below monitor refresh rate, do i still need to have v-sync enabled?

Sort the comments here by newest; the answer is in the reply to the comment below yours, as well as in the “Range” section of this article.

Short answer to your question is “yes,” but only if you never want to see tearing.

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15 hours 41 minutes ago

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SturmButcher

I don’t understand something…. Why do I need V-Sync on with G-Sync?, The last one already sync frames and If I use a frame limiter I would never need V-Sync, I don’t understand guys. I am suspecting that G-Sync+V-Sync+RTSS is causing stutter in some games…Could you please enlighten me?

I’ve explained this ad nauseam (including in this article), but I’ll try to break it down as clearly as possible yet again… Within the G-SYNC range (e.g. within the refresh rate), G-SYNC is V-SYNC, and V-SYNC is G-SYNC. The “V-SYNC” option in the “on” position with G-SYNC enabled was originally a non-optional part of G-SYNC. It was only at a later point Nvidia revealed the V-SYNC “on/off” option in the control panel so that the screen would tear with V-SYNC “off” when the framerate exceeded the G-SYNC range/refresh rate, instead of reverting to V-SYNC behavior. However, the V-SYNC option also… Read more »

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16 days 4 hours ago

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vityapapa

The Csgo input-lagg is the best g-sync off+v-sync off and fps_max 0? i have 144hz monitor.